Radio frequency identification devices generally comprise RF tags and a read-out device that is usually called interrogator or integration reader. The interrogator generates a continuous wave (CW) RF carrier that is used by the tag as a power source to modify the amplitude of the CW carrier by loading and unloading its antenna with stored digital codes. The modulated backscattering signals are then reflected back to the interrogator and demodulated, and thereby, the information stored in tags is read by the interrogator. RFIDs tags can be read through water, paint, dirt, wood, plastics, and human bodies. They are used broadly in security systems, electronic access cards, and inventory management systems.
RFIDs can also be used with sensors. In this application, typically, physical or chemical properties of an object, such as temperature, humidity, pressure, speed, pH, and acceleration, are detected as analog electrical signals. Then an Analog to Digital Converter (ADC) is employed to convert the analog signals into digital signals, which are read by the interrogator during a sampling cycle. Generally, an ADC compares the analog input voltage with a reference voltage in generating digital signals. Therefore, to obtain an accurate result, a high precision and stable reference voltage source is needed, and the variation of input voltage during sampling should be minimized. However, the power supply of RFID tags is usually generated by converting CW to direct current (DC). It is not easy to obtain a precise reference voltage without using an embedded battery cell. On the other hand, the signal sensing and A/D need extra power consumption, which needs a more powerful CW or closer operation range.
It is an object of the present invention to provide a RFID tag that is able to work with an interrogator to convert sensing values into digital signals without using A/D converters. Another object of the present invention is to provide a means to transmit the sensing information with identification (ID) codes.